This invention relates generally to dewpoint sensors and more specifically to non-optical dewpoint sensors.
Industrial power generation gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. The turbine is typically operated for extended periods of time at a relatively high base load for powering the generator to produce electrical power to a utility grid, for example. Exhaust emissions from the combustion gases are therefore a concern and are subjected to mandated limits.
Low emission combustion systems are designed to produce low emissions and high combustion efficiency while burning natural gas fuel that is assumed to be free of liquid or solid contaminants. In fact, pipeline gas is at times contaminated with condensed liquid hydrocarbons (to varying degrees) as well as other solid particulate contaminants. It is highly desirable to minimize the effects of these contaminants on gas turbine combustor performance, either by their removal or by robust combustor design. As low emission systems have become more prevalent in the field and exposed to a variety of natural gas sources while performing with lower and lower emission goals, the presence of varying amounts of liquid hydrocarbons in the fuel source has become an increasing operational issue.
The quality of natural gas supplied to gas turbines is an important variable in turbine performance. The principle component of natural gas is methane, which typically accounts for over 90% of the mass. Other components in natural gas may include heavier hydrocarbons, oils and water. In gas turbines equipped with combustors that premix fuel and air prior to ignition, the chemical composition of the gas is particularly important because of the potential for ignition to occur within the mixing zone. The effect of heavier hydrocarbons and oils in the gas stream is to lower the autoignition temperature of the mixture. Natural gas with high concentrations of these species is more likely to ignite in the mixing zone of the combustors than in an intended flame holder region.
Certain optical dewpoint meters can determine the dewpoint of either water or hydrocarbons in natural gas. One instrument uses two chilled surfaces, each with different surface properties to distinguish between the water and hydrocarbon dewpoint. A second unit uses a specially shaped mirror that is designed to detect hydrocarbon dewpoint. To measure water dewpoint, this unit employs a second ceramic sensor that measures impedance. Both instruments are rendered inoperable when the optical surface is contaminated by certain compounds such as glycols, lubricating oils, and hydrocarbons.
Non-optical instruments exist for the measurement of water dewpoint. One unit includes humidity sensors based on electrical resistivity changes of a hygroscopic polymer coating on a ceramic substrate and sensors based on the electrical capacitance changes of a thin layer of polymer between two metal electrodes. While these polymer-based sensors do not rely on optically clean surfaces, they are only useful to measure water dewpoints, and could not be used to measure hydrocarbon dewpoints.
Quartz crystal resonators (QCR) have been used to measure water humidity and dewpoint in air and other gases. Canadian patent 1,301,477 (1992) by Claude Poritier et al. describes a QCR system to measure humidity of gases. Teruko Inamatsu and Chiharu Takahashi (1985) describe a dewpoint hygrometer based on QCR for humidity measurements. These patents, however, do not address applications to other condensable gases such as organic vapors and natural gas.
D. McKeown et al. (1985) describe the use of thermally controlled QCRs for monitoring surface contamination on the NASA Spacelab I. In this reference, a non-aqueous volatile condensable material is detected under near-vacuum (low Earth orbit) conditions. Organic materials such as plasticizers and rocket exhaust products are presumed to be the source of the contamination. This system does not operate at high pressures or have a controlled amount of sweep gas pass the sensor. Nor is this device designed to determine the dewpoint of the surrounding atmosphere.
In none of the known art of using (QCRs) for dewpoint measurement has a method been described to determine the dewpoint without actually causing condensation to occur on the crystal. Such condensation can lead to fouling of the crystal by providing sites for particulates to stick to. These particulates may not be removable during the temperature cycling of the crystal.
Accordingly, there is a need in the art for an improved non-optical dewpoint sensor.